A light emitting element using a light emitting material has features such as thinness, light weight, high speed response, direct current low voltage driving, and the like, and is expected to be applied to a next-generation flat panel display. A light emitting device in which light emitting elements are arranged in matrix is said to have superiority in wider viewing angle and higher visibility over a conventional liquid crystal display device.
A basic structure of a light emitting element is a structure in which a layer containing a light emitting organic compound (a light emitting layer) is interposed between a pair of electrodes. By applying voltage to the element, electrons from one of the pair of electrodes and holes from the other electrode are transported to the light emitting layer, so that current flows. The carriers (electrons and holes) are recombined, and thus, the light emitting organic compound is excited. When the light emitting organic compound returns to a ground state from the excited state, light is emitted.
It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
Since such a light emitting element is usually formed by using an organic thin film having a thickness of, for example, approximately 0.1 μm, it is a great advantage that the light emitting element can be manufactured to be thin and lightweight. In addition, since the time from carrier injection to light emission is approximately microseconds or less, it is also one of the features that the speed of response is quite fast. Further, since sufficient light emission can be obtained at a direct voltage of several to several tens of volts, the power consumption is relatively low. From these advantages, the light emitting element has been attracted attention as a next-generation flat panel display element.
In addition, since the light emitting element is formed to have the shape of a film, planar light emission can be obtained easily by forming a large-area element. This is a feature that is hardly obtained by a point light source typified by a filament lamp and an LED or a linear light source typified by a fluorescent light. Therefore, the light emitting element also has a high utility value as a surface light source that can be applied to lighting and the like.
However, these light emitting elements have problems with durability and heat resistance, which hinders the development of the light emitting elements. Since a light emitting element is usually formed by stacking organic thin films containing organic compounds as typified by the following Non-Patent Document 1, low durability of the organic compounds and fragility of the organic thin film are considered to be causes of the problems described above.
On the other hand, there has been an attempt to form a light emitting element by using a layer in which an organic compound and an inorganic compound are mixed instead of an organic thin film. For example, the following Patent Document 1 discloses a light-emitting element using a light-emitting layer in which fluorescent organic molecules are dispersed in metal oxide. In addition, the following Patent Document 2 discloses a light-emitting element formed by stacking a layer in which organic compounds (such as a hole transporting compound, an electron transporting compound, and a light emitting compound) are dispersed in a silica matrix while being covalently bound to the silica matrix. In these references, it is reported that the durability and the heat resistance of the element is improved.
In the light emitting elements disclosed in the foregoing Patent Documents 1 and 2, the organic compound is simply dispersed in metal oxide which is insulating. Therefore, these light emitting elements have a problem that current is hard to flow (that is, the voltage needed to apply a certain amounts of current is increased) compared with conventional light emitting elements.
In these light emitting elements, since the light emission luminance is increased in proportion to applied current density, the fact that current is hard to flow leads to a problem that a voltage for obtaining a certain luminance (that is, drive voltage) is also increased. Accordingly, when the organic compound is simply dispersed in the metal oxide, increase in drive voltage and increase in power consumption are caused even if durability and heat resistance can be obtained.
In addition, in order to suppress short circuit of a light emitting element due to dust and the like, it is effective to make a film thickness of the light emitting element thicker. However, when a structure in which the film thickness is made thicker is employed as shown in the Patent Documents 1 and 2, the drive voltage is further increased. That is, in the conventional structure, it is practically difficult to make the film thickness thicker.    [Non-Patent Document 1] C. W. Tang et al., Applied Physics Letters, Vol. 51 (12), 913-915 (1987)    [Patent Documents 1] Japanese Patent Publication No. H2-288092    [Patent Documents 2] Japanese Patent Publication No. 2000-306669